1. Field of the Invention
This invention relates to a radio receiver operational checking system using the radio receiver's active signal squelch circuit to measure the operability of a radio receiver including identifying which receiver is being checked where a plurality of radio receivers are being monitored.
2. Description of the Related Art
For years, radio receivers used in aircraft, military, mobile and other, critical radiant-energy communication links have used circuitry known as "squelch" to make the receiver silent until a "threshold" signal strength is reached, at which time full listening power is activated. Receivers with squelch have a knob for the operator to adjust to the critical silencing point, or "threshold." In some receivers, the adjustment is preset and a pull-knob disables squelch entirely, giving full background noise when volume is turned up. Without the squelch circuitry, there is a continual noise in the form of loud hissing or crackling, commonly known as background noise. The noise is distracting and unpleasant; hence, the use of squelch circuitry systems.
For example, in aviation, where almost everything is duplicated, it is common practice to install two receivers, one for standby, even in small aircraft. Often, as in aircraft, a single common speaker or headphone line is used for multiple receivers, and this invention accommodates this common audio output arrangement. Although failure is unlikely, the chance exists that any one of the thousands of electronic components in each of the receivers may cause a system failure. If so, there is nothing to indicate when a squelch-silenced receiver becomes a dead receiver. This could be serious if the aircraft is being flown on instruments in the clouds, if the controller must be heard in order to keep the plane, pilot and passengers safe, or in any number of other situations where it is crucial to have an operative radio.
Some pilots and communicators disable squelch and tolerate the noise in order to know that the radio is live. Others leave the squelch control on and trust blindly to fate. Still others bungle and foul up their communication by inadvertently having the radio turned down or off.
U.S. Pat. No. 4,718,115 (Inoue), entitled "RADIO RECEIVER FOR CARRYING OUT SELF DIAGNOSIS WITHOUT INTERFERENCE," is of interest as a general reference in this case as it discloses the category of prior art where a separate diagnostic signal is introduced in the path of the incoming signal as it goes through the radio receiver. This category of prior art is distinctively different from the present invention in that the invention, which is to be described herein, periodically deactivates the squelch circuitry as a method of determining the operational status of the radio and does not use a separate diagnostic signal.